1. Field of the Invention
The present invention relates to a method of manufacturing a thermal printing head for use in a thermal recording apparatus which may be a facsimile or a printer. More particularly, it relates to a method of forming a heat accumulating layer and an insulating layer for common electrodes.
2. Description of the Prior Art
In recent years, thermal printing heads have been widely used in various recording apparatuses such as facsimiles and printers for word processors. Requirements of these recording apparatuses are those of reduction in size and in production cost. There is, therefore, a demand for a small and inexpensive thermal printing head.
Conventional thermal printing heads are manufactured using a heat accumulating layer made of a glazed glass printed and burned on an alumina ceramic substrate whose purity is 90% or above. Manufacture of such a thermal printing head contains many processes, such as purification of alumina powder, polishing, printing and burning. Accordingly, cost thereof is high.
Recently, attention has been paid to an inexpensive thermal printing head in which a heating resistor and a circuit substrate for driver ICs are integrally formed on a heat-resistant resin substrate. This technique eliminates the alumina ceramic substrate and employs, in place of the glazed glass layer, a partially patterned polyimide resin layer, as a heat accumulating layer which controls emission and accumulation of heat.
Such thermal printing heads whose components are integrally formed on the heat-resistant resin substrate are manufactured in the manner described below. First, a flat heat-resistant resin substrate, which is cut to a predetermined dimensions, is washed. Then, a copper foil is laminated on the surface of the substrate by the heating/pressing method. This copper foil is then formed into a predetermined pattern by the photolithographic process to form one common electrode and a large number of lead electrodes in such a manner that they are separated from the common electrode by a predetermined interval and disposed in a vertical direction. Next, an electrode structure to be connected to the driver ICs by the flip chip bonding is formed by first coating nickel on the copper pattern and then gold, by the electrolytic plating. Thereafter, a polyimide heat accumulating layer is formed by coating predetermined thickness a varnish-like polyimide precursor on the substrate to a predetermined thickness by a roll coater, a spin coater or a screen printer, then by drying the coated polyimide precursor. Subsequently, etching is performed on the polyimide precursor to form a predetermined pattern and then curing the polyimide precursor is performed. Next, a heating resistor is formed by first forming on the polyimide heat accumulating layer, a layer of Ni--Cr, Ta.sub.2 N, Ta--Si--O type, Ti--Si--LO type or Ni--SiO type by sputtering and then by conducting etching, such that it has a predetermined pattern. Thereafter, a wire which connects the heating resistor to the common electrode and to the lead electrodes. Subsequently, a protective film is then formed by a sputtering or plasma CVD method. Subsequently, the driver ICs are connected by means of the flip chip bonding or the like, and the substrate is then cut into predetermined dimensions. Thereafter, the individual substrate is adhered to a heat-emitting substrate by means of a double coated tape or the like. Then, a thermistor, a capacitor, a connector and a head cover are mounted, thereby completing a thermal printing head. This thermal head, whose components are integrally formed on the heat-resistant resin substrate, exhibits excellent heat efficiency, achieves low production cost and is small in size and weight as compared with the conventional one which employs a glazed alumina substrate. These features of this thermal printing head coincide with those of a thermal printing head which has been desired in industrial fields and manufacture of this thermal printing head on an industrial basis has therefore been known.
However, in the case where the density of the thermal printing head is to be increased, the number of driver ICs increases in proportion to the number of bits of the heating resistor, thereby increasing the density of a wiring pattern. Consequently, manufacture of the thermal printing head requires higher accuracy. This increases production cost.
Accordingly, it has been proposed to provide two common electrodes and thereby decrease the number of driver ICs to one half and, hence, production cost.
However, a jumper wire which crosses (jumps) the common electrode disposed close to the lead electrodes and thus connects the common electrode disposed remote from the lead electrodes to the heating resistor, must be electrically insulated from the common electrode disposed close to the lead electrodes by means of an insulating layer. This makes the manufacturing process complicated.